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SIMPLIFIED MODEL OF THE LOWER MOTOR NEURON SYSTEM

The lower motor neuron system is housed in the central and peripheral nervous system (97.1 [A,B]).
The parts of the lower motor neuron system housed in the central nervous system are the bodies of the lower motor neuron and the small segment of their axons. The bodies of the lower motor neurons are housed in the brainstem and in the spinal cord. Their axons travel briefly in the central nervous system and then exit. The segment of the axons of the lower motor neurons travel in the central nervous system and are surrounded by myelin of oligodendrite origin.
The part of the lower motor neuron system in the peripheral nervous system are the nerve, myoneural junction, muscle sensory apparatus, and muscle. The nerves are formed by the myelinated (Schwann cell origin) axons from the lower motor neurons and the axons from the sensory and autonomic neurons. The myoneural junction is the area of functional contact between the nerve and muscle. The muscle sensory apparatus consists of the neuron bodies of the propioceptive sensory neuron in the dorsal spinal ganglion, their axons, and the intrafusal muscle fibers. The axon of the propioceptive sensory unipolar neuron of the dorsal spinal ganglion sends one arm toward the spinal cord. This arm of the axon enters the spinal cord and makes a loop in the spinal cord to contact with the lower motor neuron body (Figure 97.1 [A]. The other arm travels toward the muscle and innervates the muscle spindle. The muscle spindle is the tension sensor of the muscle.

A B

Figure 97.1. Schematic representation of the lower motor neuron system [A]. The relation between relevant regions of the central nervous system and the lower motor neuron system [B]. The lower motor neuron is represented in blue. The sensory neuron is represented in dark orange. SC: spinal cord; LMN: lower motor neuron; MNJ: myoneural junction. Green squares represent myelin of Schwann cell origin. Light brown squares represent myelin of oligodendroglia origin.

LOCALIZING THE SITE OF DAMAGE USING A SIMPLIFIED MODEL IN NEONATES WITH NEUROMUSCULAR HYPOTONIA

The possible site of neuromuscular dysfunction (Figure 97.2) is determined by analyzing the clinical findings and is confirmed by neurophysiological testing or imaging studies. There are 10 possible sites.


Figure 97.2.
Schematic representation of the possible sites of neuromuscular involvement in neonates with hypotonia: (1) brain; (2) brainstem; (3) rostral spinal cord; (4) brachial plexus area of the spinal area; (5) cerebellum; (6) alpha motor neuron; (7) nerve; (8) presynaptic-myoneural junction; (9) postsynaptic-myoneural junction; (10) muscle. Green squares represent myelin of Schwann cell origin. Light brown squares represent myelin of oligodendroglia origin.


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anal sphincter weakness lack of bowel movements abnormal parasympathetic pupillary function increased gag, glabellar and jaw reflexes weakness of facial muscles seizures muscle myoneural junction nerve lower motor neuron cerebellum brachial plexus spinal cord area rostral spinal cord brainstem brain postsynaptic myoneural junction postsynaptic myoneural junction presynaptic myoneural junction nerve somatic motor neuron in the anterior horns of the spinal cord somatic motor neuron in the brainstem pause pointer over numbers and squares in lateral bar